Of the various electrostatic printing techniques, the most familiar and widely utilized is that of xerography, wherein latent electrostatic images formed on a charge retentive surface are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper. A lesser known and utilized form of electrostatic printing is one that has come to be known as direct electrostatic printing (DEP). This form of printing differs from the aforementioned xerographic form, in that the toner or developing material is deposited directly onto a non-image-charged substrate in image configuration. This type of printing device is disclosed in U.S. Pat. No. 3,689,935 issued Sep. 5, 1972 to Gerald L. Pressman et al.
Pressman et al. disclose an electrostatic line printer incorporating a multilayered particle modulator or printhead comprising a layer of insulating material, a continuous layer of conducting material on one side of the insulating layer, and a segmented layer of conducting material on the other side of the insulating layer. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of the conductive material is formed around at least a portion of an aperture and is insulatively isolated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layer while a fixed potential is applied to the continuous conductive layer. An overall applied field projects charged particles through the row of apertures of the particle modulator and the density of the particle stream is modulated according to the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charged particles impinge upon a print-receiving medium interposed in the modulated particle stream and translated relative to the particle modulator to provide line-by-line scan printing. In the Pressman et al. device the supply of the toner to the control member is not uniformly affected and irregularities are liable to occur in the image on the image receiving member. High-speed recording is difficult and moreover, the openings in the printhead are liable to be clogged by the toner.
DEP printheads such as those described in, for example, U.S. Pat. Nos. 4,647,179; 4,743,926; 4,876,561; 5,040,004; and 5,038,159 typically comprise an electrically insulative base member in which the apertures are defined, which is fabricated from a polyimide film of a thickness of one to two mils (0.025 to 0.05 mm). The most common material for the base member is a thin layer of polyimide plastic. This material and others similar thereto have several practical disadvantages. Such a thin membrane as needed for the base member will obviously be very flexible and fragile, and consequently, the base member must be mounted on a rigid precision plate. The precision plate must be very finely machined to maintain a consistent spatial relationship among the donor roll, the apertures, and the paper. The thinness and flexibility of the base member obviously presents a problem of ruggedness while the equipment is in use. These numerous practical problems add to the cost and reduce the reliability of prior art DEP printheads.
U.S. Pat. No. 4,860,836 to Schmidlin discloses a near-letter-quality DEP printhead having at least three rows of equally spaced and staggered apertures. As can be seen in that patent, however, each aperture is addressed with a single dedicated electrode for that aperture only. This arrangement is clearly expensive, and requires the placement of many delicate conductive leads on the base member, with the attendant problems of durability.
It is an object of the present invention to provide a DEP printhead which avoids many of the practical problems associated with prior art printheads.
It is another object of the present invention to provide such a DEP printhead which is made of a rigid, non-crystalline ceramic, which may be precisely shaped to conform to the shape of a donor roll, and in which multiple rows of apertures may be provided, whereby the apertures in each row are precisely spaced relative to a donor roll and a substrate.
It is another object of the present invention to provide such a DEP printhead which may be manufactured by the convenient method of photosensitive etching, for forming both the apertures and the electrodes therein.
It is another object of the present invention to provide such a ceramic DEP printhead which allows convenient installation of active electronic devices thereon.
Other objects will appear hereinafter.